In the use of explosives, the need for safety is a problem which is ever present with workers in the field. Many explosives used in the field are compounded into a finished form at the factory and shipped from there to the point of use. During shipment, special precautions must be taken. Frequently, these precautions involve limitations on the routing of the explosives and the type of carrier, etc. Further, there are limitations as to the manner in which the explosives can be shipped and stored.
As a result of these requirements regarding shipping, storage, etc., dynamite and other sensitive explosives are quite expensive to ship and store. This is reflected by a relatively high cost for the explosive material which must be borne by the ultimate user.
Some years ago, the commonly used blasting material for general industrial and commercial use, such as quarrying or earth-work blasting, or the like, was dynamite. In recent years, dynamite has been largely displaced by relatively simple mixtures of ammonium nitrate with sensitizers such as diesel oil. While the latter has met with wide acceptance, the commonly used mixtures are not without some disadvantages.
The explosive most commonly used is prilled ammonium nitrate fueled with fuel oil or similar liquid hydrocarbon, and sometimes supplemented with smaller amounts of other ingredients -- the generic term for this composition is AN/FO.
AN/FO has accounted for perhaps two-thirds or more of the industrial explosives used during 1971, up from practically zero 15 years ago. To gain such acceptance, it had to have had several factors in its favor. Paramount was economics which is a price/performance relationship; second was its ease of handling in bulk, which has lowered the cost of loading holes to an almost irreducible minumum; third was its uniformity of performance, which has permitted almost instantaneous acceptance and widespread use.
But AN/FO's uniformity is also its major disadvantage from the standpoint of bulk loading densities and resultant borehole pressures. One of the serious disadvantages of ammonium nitrate/fuel oil type explosives is excessive low density that leads to low borehole pressure. For example, AN/FO with a ratio of ammonium nitrate to fuel oil of 94/6 has a density of about 0.8 gm/cc with a corresponding borehole pressure usually less than 140 tons/sq. in. (19 kilobars). Thus with AN/FO mixtures, drilling costs are abnormally high -- and shovel costs are frequently higher than normal, because low detonation pressure causes reduced fragmentation which necessitates closer drilling of holes. Therefore, the low cost of AN/FO is somewhat offset by higher drilling and shovel costs.
There has been little impovement in AN/FO loading densities since bulk loading of holes was first done over 10 years ago. There have, however, been many attempts to overcome this disadvantage, which attempts have been unsuccessful from either an economic or a modis operandi standpoint.
Users and producers of AN/FO have tried various methods of improving bulk loading densities, such as grinding, weight additives and special downhole loading equipment. It was thought that grinding would increase the density since the finer particles could fill the void spaces between the spherical prills. But grinding has resulted in little or no improvement of density.
At first I attributed the lower field densities to the feathering action on the blasting agent by the discharge screw conveyor, and it was thought this effect could be overcome by loading material in 10 to 50 pound batches. This method resulted in a slight improvement, but with insufficient additional performance to offset handling problems.
It has been found that the reason for the inability to achieve higher bulk loading densities using ground ammonium nitrate is because of a phenomena which I call "fluffing." Because of this fluffing, the ammonium nitrate never actually has a chance to become dense and compact enough to produce the desired detonation velocity.
Fluffing occurs when ground ammonium nitrate having less than 9 percent liquid fuel therein falls to the bottom of the hole, "bounces" off of the bottom and thereby is dispersed with the air in the bottom of the hole. After bouncing, the ground ammonium nitrate settles as a fluffy mass of low (0.80 -0.85 gm/cc) density. Batches of ground AN/FO having the usual 6 percent fuel oil were dropped 20 feet into large diameter Lucite pipe and the materials were observed to rebound off the bottom and settle as a fluffy low-density mass. After discovering the problem, I set out to find a solution to this density problem while keeping the explosive non-cap-sensitive. On the assumption that wetter material would be more cohesive and result in higher densities, mixtures using 9 percent to 18 percent fuel oil were tested. These gave higher densities, but as expected, the various mixtures either detonated at low-order or failed to detonate.